1. Field of the Invention
The present invention relates to an optical disc apparatus for reading and reproducing data such as video data and audio data which are recorded on an optical disc such as a DVD (Digital Versatile Disc), and further relates to a method for adjusting levels of photo-detecting signals outputted from photo-detecting areas of a photo-detector used in the optical disc apparatus.
2. Description of the Related Art
In an optical disc apparatus, a laser beam is focused on a recording face of an optical disc such as a CD (Compact Disc), an LD (Laser Disc), a DVD, an MD (Mini Disc), and a reflected beam from the recording face is detected by a photo-detector provided on an optical pickup. The photo-detector converts photo-signals to electric signals, so that the data recorded on the optical disc can be read out and reproduced, electrically. The electric signals outputted from the photo-detector are further used for adjusting focusing of the laser beam on the recording face of the optical disc. The photo-detector used for a method utilizing astigmatism (hereinafter, called “astigmatic method”) has partitioned four photo-detecting areas and outputs four photo-detecting signals respectively corresponding to the partitioned four photo-detection areas. A signal processor generates an RF signal, a focusing error signal and a tracking error signal with using the photo-detecting signals outputted from the photo-detector.
Since the photo-detector is mounted on the optical pickup by an automated machine, a position of the photo-detector includes a positioning error on the optical pickup. Thus, levels of the photo-detecting signals outputted from the photo-detecting areas of the photo-detector are occasionally not even due to the positioning error, although the laser beam is just focused on the recording face of the optical disc. Such a phenomenon is called “unbalance of the photo-detector”. When the differences between the levels of the photo-detecting signals are larger than a predetermined error margin, the focusing error and the tracking error cannot be detected precisely. Thus, it is necessary to adjust the levels of the photo-detecting signals outputted from the photo-detecting areas of the photo-detector for compensating the positioning error.
For adjusting the levels of the photo-detecting signals outputted from the photo-detecting areas of the photo-detector to be even, signals such as the focusing error signal or the tracking error signal, which are generated by processing the photo-detecting signals, are conventionally used. It, however, is substantially impossible to adjust the levels of the photo-detecting signals precisely even, since the focusing error signal or the tracking error signal includes error component due to the unbalance of the photo-detector.